What it shows
Heavy bags can change the way a student stands, with the head and shoulders leaning forward. Measuring bag weight as a share of body weight and checking posture in side photos gives data to discuss a safe bag load.
The science
The body stays balanced when its centre of mass lies above the feet. A heavy bag at the back pulls the centre of mass backward, so the body leans forward to compensate. The angle between the vertical and the line from ear to shoulder shows the lean, and a lighter, centred bag usually reduces it.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A bathroom scale or a kitchen scale
- A phone camera and a marker
- A plumb line made from thread and a nut
- Chart paper and a protractor
- Permission slips from parents
- A notebook
Safety
For this project: Take part only with permission of parents and teachers. Photographs must not show faces or names. This is not medical advice, and anyone with back pain should see a doctor.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Weigh each volunteer and their packed bag, and write the bag as a percentage of body weight.
Take a side photo of the volunteer standing naturally with an empty bag, with a plumb line hanging beside them.
Take a second photo with the full bag on both shoulders, and a third with the bag on one shoulder.
Draw a line from the ear to the shoulder on each photo and measure its angle from the plumb line.
Make a table of bag weight percentage and leaning angle.
Plot a scatter graph of angle against weight percentage.
Write what you see in the graph and what limits it, like different heights.
Make a poster with tips such as using both straps and carrying only what is needed.
What you should see
A scatter graph that usually shows a bigger forward lean for heavier bags, and a poster with simple tips for sharing the load.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the centre of mass?
The point where the weight of a body acts.
2Why do people lean forward with a heavy bag?
To keep their centre of mass over their feet.
3Why use both shoulder straps?
It spreads the load and keeps the body more symmetrical.
4Why are faces hidden in the photos?
To protect privacy.
Take it further
Compare a school bag with a rolling trolley bag and note the leaning angle.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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